Certain properties of gypsum (calcium sulfate dihydrate) make it very popular for use in making industrial and building products, such as gypsum wallboard. Gypsum is a plentiful and generally inexpensive raw material which, through a process of dehydration and rehydration, can be cast, molded or otherwise formed into useful shapes. The base material from which gypsum wallboard and other gypsum products are manufactured is the hemihydrate form of calcium sulfate (CaSO4.½H2O), commonly termed “stucco,” which is produced by heat conversion of the dihydrate form of calcium sulfate (CaSO4.2H2O), from which 1-½ water molecules been removed.
Conventional gypsum-containing products such as gypsum wallboard have many advantages, such as low cost and easy workability. Various improvements have been achieved in making gypsum-containing products using starches as ingredients in the slurries used to make such products. Pregelatinized starch, for example, can increase flexural strength and compressive strength of gypsum-containing products including gypsum wallboard. Known gypsum wall board contains board starch at levels of less than about 10 lbs/MSF.
It is also necessary to use substantial amounts of water in gypsum slurries containing pregelatinized starch in order ensure proper flowability of the slurry. Unfortunately, most of this water must eventually be driven off by drying, which is expensive due to the high cost of the fuels used in the drying process. The drying step is also time-consuming. It has been found that the use of naphthalenesulfonate dispersants can increase the fluidity of the slurries, thus overcoming the water demand problem. In addition, it has also been found that the naphthalenesulfonate dispersants, if the usage level is high enough, can cross-link to the pregelatinized starch to bind the gypsum crystals together after drying, thus increasing dry strength of the gypsum composite. Trimetaphosphate salts have not in the past been recognized to affect gypsum slurry water requirements. However, the present inventors have discovered that increasing the level of the trimetaphosphate salt to hitherto unknown levels in the presence of a specific disperant makes it possible to achieve proper slurry flowability with unexpectedly reduced amounts of water, even in the presence of high starch levels. This, of course, is highly desirable because it in turn reduces fuel usage for drying as well as the process time associated with subsequent water removal process steps. Thus the present inventors have also discovered that the dry strength of gypsum board can be increased by using a naphthalenesulfonate disperant in combination with pregelatinized starch in the slurry used to make the wallboard.
Conventional gypsum wallboards have adequate strength for working, and meet standard test requirements such as nail pull (77 lb) and core hardness (11 lb). However, conventional wallboards are heavy, typically weighing up to 1600-1700 lb/MSF. If a way could be fund to produce a high strength gypsum wall board in which board weight (and density) is significantly reduced, without adversely affecting nail pull and hardness characteristics, this would represent a useful contribution to the art.
It is also known in making gypsum wallboard that bonding layers can be used to promote adherence or bonding of the paper cover sheets to the set gypsum core. Typically, these bonding layers are relatively thick, ranging from about 7 mils to about 25 mils, even up to 50 mils. Thinner bonding layers would be expected to be more difficult to apply and to present other drawbacks. Unfortunately, the use of these thick bonding layers can decrease the core hardness in the finished wallboard. Finished densities in these dried bonding layers range from greater than about 70 pcf to about 90 pcf. The term “pcf” is defined as pounds per cubic foot (lb/ft3). If a way could be found to make a low density set gypsum board using thinner, lighter bonding layers, without sacrificing core hardness or other important board properties, this would represent a useful contribution to the art.